This invention relates to control apparatus of a flying shear which cuts a strip material running along a process line.
A flying shear of this type is used to cut the leading end and the trailing end as well as intermediate points of a metal strip produced by a rolling mill, for example, and has had a construction as diagrammatically shown in FIG. 1. As shown, two cutting blades 1.sub.a and 1.sub.c are disposed in a common plane and provided with meshing gears 5.sub.a and 5.sub.c on shafts 1.sub.sa and 1.sub.sc so as to rotate in opposite directions. An electric motor 3 is provided for driving the cutting blades 1.sub.a through gears 6.sub.a, 6.sub.b and a clutch 2. The speed of the motor 3 is controlled by a signal generated by a tachometer generator 8 mounted on the motor shaft 3.sub.s so that the motor speed is always proportioned to the strip speed. A brake 4 is mounted on the shaft 1.sub.sc of the cutting blade 1.sub.c so as to apply braking force upon both cutting blades 1.sub.a and 1.sub.c. A rotational angle detector 7 is mounted on the rear end of the shaft 1.sub.sc.
During the operation of the flying shear, cutting is commenced by disengaging the brake 4 and actuating the clutch 2 to transmit rotation of the motor 3 to the upper and lower cutting blades 1.sub.c and 1.sub.a. As the upper and lower cutting blades 1.sub.c and 1.sub.a meet each other and the cutting is completed, the clutch 2 is disengaged and thereafter the brake 4 is actuated so that the cutting blades 1.sub.c, 1.sub.a are brought to a standstill at the original position.
The flying shear of the type mentioned above is generally called clutch-brake type shear. Since the motor 3 is kept rotating regardless of whether the cutting is being effected or not, a flywheel 9 can be mounted on the motor shaft 3.sub.s, and the energy stored in the flywheel 9 can be utilized for the cutting, so that the capacity of the motor 3 can be reduced compared with the start-stop type shear wherein the cutting blades 1.sub.c and 1.sub.a are always kept coupled to the motor shaft 3.sub.s and the motor 3 is rotated only during the cutting. The clutch 2 and the brake 4 are usually pneumatically operated and controlled by electromagnetic valves in view of mechanical convenience and desired operational speed.
FIG. 2 shows the above-mentioned flying shear incorporated into a process line. A speed control apparatus 10 controls the speed of the motor 3 in response to a reference signal correlated with the movement of the strip and a feedback signal from the tachometer generator 8 which detects the speed of the motor 3. A cut start instruction circuit 11 receives a reference signal correlated with the strip speed and a strip detection signal from a strip detector 13 and performs operation on these signals to produce an output signal for instructing the commencement of the cutting. The operation performed by the cut instruction circuit may vary according to the particular requirement. An example of such operation is described below.
Assuming that L represents the distance from the strip detector 13 to the cutting point (where the cutting is conducted) of the shear, l.sub.s the locus of tip of the cutting blades 1.sub.a, 1.sub.c from the rest point to the cutting point, l the desired length of each cut, v the speed of the strip, k the speed lead rate of the cutting blades 1.sub.a, 1.sub.c relative to the strip speed (at the instance of cutting), .alpha. the rate of acceleration at which the cutting blades 1.sub.a, 1.sub.c are accelerated by the clutch torque during the time since the clutch 2 is actuated and until the cutting blades 1.sub.a, 1.sub.c are synchronized with the motor speed, and t.sub.d the dead time which elapses since the cutting instruction is given and before the cutting blades actually begin to move. When the leading end or the trailing end of the strip is detected by the strip detector 13, integration operation of the strip speed is commenced. As the integral, i.e., the distance over which the strip has so far travelled satisfies the following equation, the cut start instruction signal is generated. ##EQU1##
In the equation (1), the sign before "l" is plus where the leading end of the strip is detected, and the sign before "l" is minus where the trailing end of the strip is detected. The third term of the right side: ##EQU2## is the distance over which the strip proceeds during the time since the cutting blades begin rotation and before it reaches the cutting point. The fourth term of the right side: EQU v.multidot.t.sub.d
is the distance over which the strip proceeds during the dead time. When the cut start instruction is generated, the brake 4 is disabled and then the clutch 2 is actuated, and accordingly the cutting blades 1.sub.a, 1.sub.c are rotated to cut the strip.
When the cutting is completed, the rotational angle detector 7 (usually comprising a set of limit switches) detects a mechanically predetermined point at which the clutch 2 is to be disengaged and another mechanically predetermined point at which the brake 4 is to be actuated, and thereby the clutch 2 is disengaged and the brake 4 is thereafter actuated to stop the rotation of the cutting blades. The points at which the rotational angle detector 7 generates the instruction for disengaging the clutch and the instruction for actuating the brake are both fixed. As a result, when the motor speed is changed in conformity with a change of the strip speed, the angle over which the cutting blades rotate since the instruction for disengaging the clutch and before the cutting blades actually come to a stand is not constant and the cutting blades rest at varying positions. It will be understood from the equation (1) above, that the variation of the rest position means the variation in the locus l.sub.s of the tips of the cutting blades from the rest position to the cutting position, and hence it affects the length of each cut of the strip.
To eliminate such defects and to enhance the accuracy of the length of the cut of the strip, a conventional technique employs a system wherein the detection points at which the rotational angle detector 7 produces the instruction for declutching and the instruction for braking can be selected among several positions, depending on the speed of the strip. The rotational angle detector comprises, for example, a rotary cam limit switch having six pairs of contacts, the operating angle of each pair being adjustable independently of each other. The detection points can be selected among the corresponding positions of the pairs of contacts, depending on the speed. If the different detection points are required depending upon whether the speed is high, middle or low (as is usual) six pairs of contacts are needed for the control. Consequently, if additional pairs of contacts are required for other sequential controls, an additional rotary cam limit switch has to be provided.
The provision of two rotary cam limit switches will increase the length of the cutting blade shaft. Since the acceleration of the cutting blade shaft is great, the torsion of the shaft is problematical and the rotary cam limit switch may be damaged. It will be seen that there is a limit to the number of limit switches which can be provided, and hence the rest point of the cutting blades is undesirably affected by the change of the line speed.